Liquid crystal polymers are a family of materials that exhibit a highly ordered structure in the melt, solution, and solid states. They can be broadly classified into two types; lyotropic, having liquid crystal properties in the solution state, and thermotropic, having liquid crystal properties in the melted state. For convenience, and unless otherwise stated, the term "liquid crystal polymer", as used hereinafter, refers to thermotropic liquid crystal polymers, and is meant to include polymer alloys having a liquid crystal polymer component as well as liquid crystal polymers alone.
Most liquid crystal polymers exhibit excellent physical properties such as high strength, good heat resistance, low coefficient of thermal expansion, good electrical insulation characteristics, low moisture absorption, and are good barriers to gas flow. Such properties make them useful in a broad range of applications in the form of fibers, injection molded articles, and, in sheet form, as electronic materials for printed circuit boards, packaging, and the like.
However, many of the physical properties of liquid crystal polymers are very sensitive to the direction of orientation of the liquid crystal regions in the polymer. The ordered structure of the liquid crystal polymer is easily oriented by shear forces occurring during processing and highly aligned liquid crystal chains can be developed that are retained in the solid state, and result in highly anisotropic properties. Additionally, in many cases, articles formed of liquid crystal polymers, in which the liquid crystal polymer is anisotropically oriented, exhibit surface roughness and nonuniformity of thickness in the direction perpendicular to the direction of orientation. The surface topography is somewhat akin in appearance to that of a plowed field, having parallel ridges and valleys which are relatively uniform in the longitudinal direction of orientation, but can have considerable roughness and thickness variability in the transverse direction. This can be highly desirable for certain products, for example, in filaments, fibers, yarns, and the like. Such properties resulting from anisotropic orientation are often not desirable, however, in products having planar forms, such as tape, films, sheet, and the like; and in which smooth surfaces and uniform thickness may be important.
A number of methods are used to produce liquid crystal polymer materials in planar forms that have more balanced, less anisotropic properties. These include the use of multilayer flat dies which are oriented such that they extrude overlapping layers at intersecting angles, use of static mixer-agitators at the die inlets, and the like. More recently, dies having rotating or counter-rotating surfaces have become known in the art and successfully used. These extrusion techniques, used separately or in combination with other methods known in the art, such as film blowing, can produce liquid crystal polymer film and sheet that are multiaxially-oriented, that is, oriented in more than one direction, and have more balanced physical properties.
A characteristic of these methods is that locally, for example, at the surfaces of the sheet or film, the molecules are oriented in the planar x-y directions by shear imparted at the extrusion surfaces. In the z-direction, i.e., throughout the thickness, the x-y orientation of the molecules will change progressively from the orientation at one surface to the orientation at the other surface of the planar form. A drawback to these methods is that when attempting to make multiaxially-oriented films, the forces imparted by the extrusion surfaces to the liquid crystal polymer are exerted in opposing directions and the formation of pinholes, tears, and other imperfections, for example, separation of surface layers of the film can take place.
A method to reduce uneveness of thickness is suggested in Japanese Laid-Open Patent Application No.4-166309, which discloses calendering a thermotropic liquid crystal polymer sheet or film between rolls heated to a temperature greater than the glass-transition temperature and less than the melting temperature of the liquid crystal polymer. Compared to ordinary thermoplastic polymers, however, liquid crystal polymers exhibit relatively little softening at temperatures less than the melt temperature and high roll pressure, in the range 100-3000 kg/cm, is required by the process.